Cellular wireless communication systems are designed to serve many mobile station's distributed in a large geographic area by dividing the area into cells. At the center of each cell, a base station is located to serve mobile stations located in the cell. Each cell is often further divided into sectors by using multiple sectorized antennas (The term “sector” is used both conventionally and in this document, however, even when there is only one sector per cell.) In each cell, a base station serves one or more sectors and communicates with multiple mobile stations in its cell. Data may be transmitted between the base station and the mobile stations using analog modulation (such as analog voice) or digital modulation (such as digital voice or digital packet data).
A base station includes devices needed to transmit and receive signals to and from mobile stations, which typically include modems, up/down converters, analog-to-digital converters (ADCs), digital-to-analog converters (DACs), filters, low noise amplifiers (LNAs), power amplifiers, and transmit and receive antennas. A base station also includes devices to transmit and receive mobile station's signals as well as other control signals to and from other systems such as a base station controller that controls multiple base stations.
A Radio-Frequency (RF) carrier in a sector can handle up to a certain amount of data traffic, which is referred to as the capacity per carrier per sector or simply capacity. In general, the capacity is different in the forward and in the reverse links. Because base stations normally overlap RF coverage in a geographical area, and service hundreds or thousands of mobile station users, spectral interference among base and mobile stations is usually the dominant limitation of total system capacity. Thus, RF interference mitigation often becomes an important design objective for radio access networks. RF interference reduction techniques include frequency, time, or code domain separation of mobile and base station RF signals. A combination of these basic schemes can also be employed. For Code Division Multiple Access (CDMA) cellular networks, each base station sector operates at the same RF frequency, but with a different pseudorandom noise (PN) code as the base modulation of the carrier. The PN codes among co-located sectors are chosen as to have very low cross-correlation products when processed by the mobile station receiver de-correlation circuits. For this technique to work properly, precise timing and synchronization is required among the base stations to preserve the orthogonal cross-correlation properties of these PN codes.
In a conventional base stations, timing and synchronization functions are provided by a specialized GPS receiver that is incorporated into the base station equipment. This specialized GPS receiver typically provides time-of-day messages, a precise one or two second timing pulse, and an ultra-stable frequency reference. Because all base stations in a network derive their synchronization from the GPS network, excellent timing synchronization is achieved. In this conventional synchronization scheme, each base station is provided with a separate GPS receiver and must be located in a facility with access to an external GPS antenna system with visibility to the GPS satellites. As cellular networks become smaller with more numerous base stations per network, the cost of providing a separate GPS receiver at each base station can become considerable.